Direct Measurement of the Local Glass Transition in Self-Assembled Copolymers with Nanometer Resolution

Nanoscale compositional heterogeneity in block copolymers can impart synergistic property combinations, such as stiffness and toughness. However, until now, there has been no experimental method to locally probe the dynamics at a specific location within these structured materials. Here, this was achieved by incorporating pyrene-bearing monomers at specific locations along the polymer chain, allowing the labeled monomers' local environment to be interrogated via fluorescence. In lamellar-forming poly(butyl methacrylate-b-methyl methacrylate) diblock copolymers, a strong gradient in glass transition temperature, T_g, of the higher-T_g block, 42 K over 4 nm, was mapped with nanometer resolution. These measurements also revealed a strongly asymmetric influence of the domain interface on T_g, with a much smaller dynamic gradient being observed for the lower-T_g block.